Amplifiers are used today in a wide range of applications for amplifying different signals. In for example radio communications systems a power amplifier typically amplifies a radio frequency (RF) signal in a transmitter prior to transmission to a receiver.
One basic problem in power amplifiers is when they are operated at or close to their saturation levels. At such high power demand levels (close to the saturation power limit of the amplifiers), amplifiers may experience drastic changes in amplitude and phase characteristics, increasing the intermodulation (IM) distortion. In addition, amplifiers in radio communications systems are often required to simultaneously amplify several different carrier signals. As a result, nonlinear intermodulation products are generated which may interfere with the processing of the transmitted signals.
FIGS. 1A and 1B schematically illustrate the amplitude (gain) and phase transfer characteristics of a typical power amplifier at different (input) power levels, respectively. As is seen in the diagrams, the transfer characteristics consist of a linear part 50; 70 followed at high drive levels (over the nonlinear power point PS) by a nonlinear portion 60; 80 or complex gain (amplitude gain or phase) compression. This nonlinear amplifier behavior can, thus, be decreased by reducing the power output of the amplifier. However, such power output reduction reduces the signal levels and transmission efficiency.
As a consequence, the signals provided to the amplifier are preferably processed to alter them to account for the nonlinear (intermodulation) effects. An example of such a signal processing is the predistortion technique. In this technique, the signal provided to the amplifier is altered by generating a compensation characteristic that is the opposite to the transfer characteristics of the amplifier, illustrated in FIGS. 1A and 1B, in both amplitude and phase, and this altered signal is provided to the amplifier. As a consequence, the linear operation portion 50; 70 is preferably extended into higher power levels and the intermodulation products are removed or at least reduced.
The patent document U.S. Pat. No. 6,326,843 discloses an analog reflective predistorter employed for compensating for phase and amplitude variations imposed on an input signal by one or more nonlinear transfer characteristics of a power amplifier operating at or near saturation. This predistorter includes a hybrid having signal input and output terminals and two relative phase terminals separated by 90 degrees. A respective compensation circuit is connected to each of the terminals. Such a compensation circuit includes a Schottky diode pair in parallel with a pair of resistances and a capacitance. A respective bias network is coupled to the input and output terminals of the hybrid for setting bias levels so that the compensation characteristics of the compensation circuit are started or initiated at the nonlinear behavior point (PS in FIGS. 1A and 1B) of the amplifier. In addition, the value of the capacitance in the compensation circuit and the length of the line between the diode pair and resistances determine the phase expansion of the predistorter. The amplitude predistortion of the predistorter is determined by the value of the resistances.